The classification of shared/dedicated protection is done based on the definition of shared protection as per ITU-T G.803/G.8411.e. “A protection architecture using m protection entities shared among n working entities (m:n). The protection entities may also be used to carry extra traffic when not used for protection”
The ‘n’ working entities can be either part of the same network element in a form of a link or an STM path entity which are the cases of 1:N MSP and 1:N SNCP respectively or part of different network elements in the case of MSSPRing (Multiplex Section Shared Protection Ring).
A shared protection ring comprises of N network elements or nodes forming a ring via interconnected fibers in which each of them have some reserved work capacity and some reserved protect capacity i.e. in state of the art 2-fiber systems, for a total capacity of X, X/2 is each reserved for work and protect, wherein the reserved protect capacity is used to carry the work traffic of any of the N nodes against any failure anywhere in the ring. In case the protect capacity is not required to protect any of the work traffic in the ring, it carries a lower priority extra traffic.
The term “shared” implies that for a work traffic there is no dedicated protect which is always available. In simple terms the protect capacity is shared and might be used by any of the network elements forming the protection ring when required (based on where the failure has happened in the ring).
Depending on which of the spans in the ring a failure(s) has happened, the protected work traffic which gets carried on the network element's protect capacity can be one of its own work traffic or the traffic of one of the N network elements whose work traffic is affected due to the failure.
FIG. 1 shows a 2-fiber architecture which explains the concept of shared protection as an example. In FIG. 1, 4 nodes (A, B, C, D) are shown to be part of a 2 fiber MSSPRing ring. Two work circuits have been shown: one from A-B and the other C-D.
FIG. 2 shows that a failure between the link C-D leads to protection of the circuit going through C-D span being carried via the long path D-A-B-C. Hence, the reserved protect bandwidth of D-A, A-B and B-C links are used to carry the protected work traffic across links C-D.
FIG. 3 shows that a failure between the link A-B leads to protection of the circuit going through A-B span being carried via the long path A-D-C-B. Hence, the reserved protect bandwidth of D-A, D-C and B-C links are used to carry the protected work traffic across links A-B.
As shown in FIGS. 2 and 3, in both the scenarios, the links A-D and B-C are used to carry different protected work traffics, in one case that of C-D and in the other case that of A-B. Hence, the location of failure decides what work traffic is carried on the reserved protect bandwidth. Since, the reserved protect bandwidth is used for protection of the work traffic for the whole ring, hence the term shared is being used.
As shown in FIGS. 2 and 3 as examples, the work and the protect division is done in the same link/fiber. For a signal rate of X, X/2 each is reserved for each of the work and protect. This is the case of 2-fiber MSSPRing/BLSR.
In case the protect is all together carried on a different fiber, the protection mechanism becomes a 4 fiber MSSPRing/BLSR. Here, two fibers are designated to contain work traffic and the other two to carry the protect traffic. FIG. 4 illustrates the same. 4 fiber MSSPRing/BLSR has an additional span protection in addition to the ring protection provided by 2 fiber MSSPRing/BLSR as illustrated by FIGS. 5 and 6.
In SDH/SONET networks the most common and fully standardized protection schemes (recommended as per ITU-T G.841, Bellcore GR1230, Bellcore GR 1400) are:                1+1 SNCP/UPSR        1+1 MSP/APS        1:N MSP/APS        For STM4/OC12 and above rates 2-Fiber MSSPRing/BLSR        For STM4/OC12 and above rates 4-Fiber MSSPRing/BLSR        
1:1/1:N SNCP schemes which can carry extra traffic are not standardized and are under further study. The reason can be that, running SNCP protocol on individual K-bytes for HO VCs and LO VCs can incur too much overhead and hence would result in high switching times in higher rate STM/OC interfaces where the number of such VCs can be huge in number. Hence, the only widespread used SNCP is the 1+1. Hence, extra traffic can't be carried and shared protection can't be achieved by using 1+1 SNCP/UPSR in a ring topology. 1:N MSP/APS schemes though can carry extra traffic but are for linear topology.
The advantages which MSSPRing/BLSR hence caters to in such cases are:                Much efficient bandwidth utilization in ring topologies because of the ‘shared’ concept as compared to 1+1 SNCP/UPSR. Here, the whole ring shares the protect bandwidth.        Extra traffic can be carried across a ring which is not possible in 1+1 SNCP/UPSR.        Faster switching times as compared to UPSR/SNCP as the signaling protocol runs at the multiplex section rather than at each virtual container.        
The basic premise for the case of 2-Fiber MSSPRing/BLSR is that the number of AU4s/AU3s/STS-1s must be even in number in a STM/OC interface. This in turn means that for an STM-1/OC-3 interface 2-Fiber MSSPRing/BLSR in accordance with the prior art cannot work as the number of switch units in AU4 mapping is ‘1’ and for AU3/STS-1 mapping it is ‘3’.
Hence, the advantages of 2-fiber MSSPRing/BLSR cannot be harnessed for a STM-1/OC-3 interface i.e. no shared protection ring and no extra traffic in a ring topology.
It must be noted that for 4-fiber MSSPRing/BLSR on STM-1/OC-3 interfaces there is no problem as there is no concept of a division of bandwidth on the same link unlike the 2-fiber case. The protect channels are part of a separate link altogether. But with 4-fiber two explicit links are required to be reserved as protect which requires more number of interfaces in the network element, i.e. more cost is involved.